SynCore.RTM., sold by The Dexter Corporation, Adhesive & Structural Materials Division, Pittsburgh, Calif. 94565 U.S.A., is a syntactic foam film that takes the place of more expensive prepreg plies in stiffening critical structures. This isotropic foam is a composite material consisting of preformed microballoons in a thermosetting matrix resin. A wide variety of preformed microballoons and matrices can be combined to make SynCore.RTM. materials. Glass is the most common microballoon material of construction, but quartz, phenolic, carbon, thermoplastic and metal coated preformed microballoons have been used. Epoxies curing at 350.degree. F. (177.degree. C.) and 250.degree. F. (121.degree. C.) are the most common thermosetting matrix resins, but matrices of bismaleimide (BMI), phenolic, polyester, PMR-15 polyimide and acetylene-terminated resins have been used to produce SynCore.RTM. syntactic foams. As a result of the variety of materials that successfully make SynCore.RTM., they are tailorable to a variety of applications. There is a version of SynCore.RTM. available that will co-cure with all known available heat-cured composite laminating resins. Syncore.RTM. allows sandwich core concepts to be used in a thinner dimension than previously possible. The thickness limit on honeycomb cores is approximately 0.125 inch. Syncore.RTM. is available in 0.007 to 0.125 inch (0.18 mm to 3.2 mm) thicknesses but can be made in thinner or thicker sheet forms. Other core materials such as wood and sheet foam can be made thin, but are not drapable and generally require an expensive/heavy adhesive film to bond to the partner composite components. In addition, Syncore.RTM. possess excellent uniformity in thickness which provides the ability to assure quality for the composite in which it is used as a component. Syncore.RTM. is typically used to replace prepreg plies where the intent is to increase stiffness by increasing thickness.
Designing with Syncore.RTM. is straightforward because all of the analysis methods that apply to other core materials such as honeycomb apply to it. Flexural stiffness of flat plates and beams increases as a cubic function of thickness allowing a lighter, stiffer lamination than could be made from prepreg plies alone. Since Syncore.RTM., on a per volume basis, typically costs less than half of a comparable carbon prepreg, it also leads to a lower cost lamination. This is illustrated by the following:
1. Adding one ply of 0.020 inch Syncore.RTM. and eliminating one ply of prepreg does not change the weight or cost significantly, but nearly doubles the flexural rigidity.
2. Adding one ply of 0.020 inch Syncore.RTM. and eliminating three plies of prepreg sharply decreases the cost and weight with a small decrease in rigidity.
3. Adding one ply of 0.040 inch Syncore.RTM. and eliminating three plies of prepreg provides lower weight, cost and sharply increases rigidity.
4. The introduction of unidirectional tape allows a further increase in performance at lower cost and weight at nearly the same thickness.
5. A hybrid tape/fabric/Syncore.RTM. construction gives a very attractive set of weight and cost savings coupled with a 3.4 times increase in flexural rigidity.
Syncore.RTM. has been recommended for thin composite structures in any application where flexural stiffness, buckling, or minimum gauge construction is used. It has been shown to save weight and material cost in carbon fiber composites. It has been offered to save weight at approximately the same cost in the case of glass fiber composites. Illustrative applications are covered in U.S. Pat. No. 4,861,649, patented Aug. 28, 1989, and U.S. Pat. No. 4,968,545, patented Nov. 6, 1990.
The manufacturing methods for employing Syncore.RTM. are very similar to those used for prepregs. Because it is not cured, it is tacky and very drapable when warmed to room temperature and is easier to lay-up than a comparable prepreg ply. It can be supplied in supported forms with a light weight scrim to prevent handling damage when it is frozen. It requires cold storage like prepregs, usually 0.degree. F. (-17.7.degree. C.) or below. The various Syncore.RTM. materials typically have a room temperature out-time that is much longer than their companion prepregs. Syncore.RTM. is less sensitive to cure cycle variations than prepreg making the controlling factor the composite cure cycle selection. It will cure void free under full vacuum or low (e.g. about 10 p.s.i.) autoclave pressure. It has been cured at up to about 150 p.s.i. without exhibiting balloon crushing.
In a typical application, a sandwich of Syncore.RTM. and prepreg, such as a thicker layer of Syncore.RTM. between two thinner layers of prepreg, are held together under heat and pressure to cure the structure into a strong panel. Typical sandwich constructions of this nature are shown in U.S. Pat. Nos. 4,013,810, 4,433,068 and 3,996,654. Such composite structures typically are produced in flat sheets and in separatable molds to obtain various desired shapes.
Though Syncore.RTM. will cure void free under significant reduced pressure or when put under pressure, it would be desirable to avoid those costly conditions to achieve void reduction. It would be desirable to have a material that has the properties of Syncore.RTM. but achieves void free construction without costly full vacuum operations or low autoclave pressure systems. These methods are typically batch type operations that materially add to the cost of making the composite.
There are certain applications in which it is desirable to have the properties of a uniform thin drapable syntactic foam film in processing the formation of a laminated composite, yet have the capacity to autogenously expand so as to fill any void space existing in the composite's structure so as to minimize the effects of macro and micro void defects at interlaminate interfaces.
These interlaminar interfacial micro or macro void spaces are magnified by the irregularity of the reinforcing layer of the composite structure. For example, if the composite is of a layer of prepreg-derived carbon fiber reinforced thermosetting resin material, bonded to a syntactic foam, such as a Syncore.RTM. thin uniform film, the layer containing the prepreg-derived material will have an irregularly shaped surface and the Syncore.RTM. layer will have a relatively smooth uniform surface. Though the Syncore.RTM. is tacky and drapable, it is incapable of filing in all of the irregularities of the prepreg-derived layer. Application of a full vacuum or the use of a low pressure autoclave can be used to significantly reduce the void space, but complete avoidance of micro voids is not readily achievable. Also, conforming Syncore.RTM. to the irregular surface causes transfer of the irregularity to the opposite surface of the Syncore.RTM. film. Such surface irregularity transfer may be avoided by sandwiching the Syncore.RTM. film using heat and pressure, such repositions the film's matrix resin and the microspheres so that the film within the sandwiched structure loses its original uniformity.
It would be desirable to be able to adequately bond a syntactic foam thin film to an irregular surface.sup.1 and fill the defects in the surface without transferring the shape of the defects to the unbonded side of the film. It would also be desirable to be able to adequately bond a syntactic foam thin film to a surface and, without the use of vacuum or low pressure autoclaves, fill the micro voids with the syntactic foam without repositioning the film's matrix resin and microspheres. FNT .sup.1 Such a surface is one that may contain undulations, cracks, large pores, warpage, and the like defects.
An advantage of Syncore.RTM. for many applications resides in its uniformity of distribution of the microsphere throughout the matrix resin. Such microspheres remain essentially intact throughout the cure cycle. As a result, it is not possible to have the microspheres concentrate at one or more surfaces, or one or more other locations in the final composite. It would be desirable to have a drapable thin film, having the handling qualities of Syncore.RTM., but which would allow the production of a syntactic foam having a controllable density gradient that accommodates specific end use applications.
There are a number of applications in which a thin film syntactic foam could serve as a seal to preclude the passage of gases and liquids. In some applications, the seal could be subjected to abrasion forces. It would be desirable to have a thin film syntactic foam that can be applied in a manner that allows it to be a sealant to gas or liquid flow in a confined space and be able to withstand abrasive forces.
There is a body of technology directed to fabricating expandable thermoplastic resinous material. For example, U.S. Pat. No. 2,958,905, patented Nov. 8, 1960, is directed to a method of making foam structures from particulate expandable granular thermoplastic resinous material containing in the particles a blowing agent for further expansion of the particles. A considerable number of thermoplastic resins are described as suitable for this purpose. The blowing agents are the conventional ones recommended for that application. The expandable granular thermoplastic resinous material may be admixed with a thermosetting resin to generate on curing the exotherm needed to expand the expandable granular thermoplastic resinous material. The resulting mass can be poured into a mold to make a number of products. The patentees indicates that the expandable granular thermoplastic resinous material can be formed in the presence of non-expandable filler materials such as staple fibers from a variety of sources, and the mixture fed to a mold for forming an expanded product. The resulting foamed product may be designed to adhesively bond to a fabric layer for reinforcement of the foamed product. The density of the foamed product can be controlled by the amount of the expandable material fed to the mold. According to the patentees, starting at column 12, lines 5 et seq., molded products are formed by charging the mold "with the expandable material in any desired manner including manual filling or pneumatic conveyance thereof." According to the description at column 12 relating to FIGS. 3 and 4 (see column 12, lines 16-32):
"a considerable occurrence of void and hollow spaces occurs between the charged expandable beads 21 in the mass to be fabricated, each of which (in the case of preexpanded material) is a foam structure containing a plurality of internal cells or open spaces. When the liquid exothermus substance is added between such interparticle voids, the heat from its spontaneous self reaction causes the beads to expand whereby, as illustrated in FIG. 4, the expanded and fabricated particles 22 force out a substantial portion (and frequently most) of the exothermus substance excepting for a minor quantity of reacted material 23 which remains, frequently as an interlaced and interlinking network between the expanded particles to assist in holding the expanded, cellular foam particles together." (Emphasis added)
U.S. Pat. No. 2,959,508, patented Nov. 8, 1960, describes another variation of using expandable thermoplastic particles. In this patent, the unexpanded particles and the exothermus substance, such as an epoxy resin, are first mixed and then poured into the mold to form a composite foam of the two when the exothermus substance heats up the mixture and causes the blowing agent to volatilize. The patent also suggest the use of chemical blowing agents in combination with the use of expandable thermoplastic particles.
A commercial version of the expanded thermosetting resins is used for "core splicing." Core splicing involves the use of a thin film epoxy resin intumescent adhesive of the pourable or film type that is maintained between release surfaces, such as polyethylene film and release paper then applied in this condition to the composite core structure for adhesive bonding of the structure. Expansion is effected by the presence of solid blowing agents in the resin composition. It splices together irregular surfaces. It is deposited on one surface to be bonded, and the other surface is brought into contact with the adhesive coating. Cure of the adhesive unifies the two parts. One such product is Hysol.TM. EA 9814NA, sold by The Dexter Corporation, Hysol Aerospace Products, Adhesives & Structural Materials Division,.
Thermosetting resins have had blowing agents incorporated in them (see U.S. Pat. No. 3,322,700, patented May 30, 1967) to form expanded molded products and recently, such types of resin systems have included preformed microspheres in the formation of partial syntactic foam films. These expanded thermosets comprise a more open cellular structure unlike that of syntactic foams, and the inclusion of preformed microspheres does not alter that condition.
There are commercial molding processes that utilize tacky sheets of thermosetting resins and reinforcing material. One such process involves the compression molding of sheet molding compounds ("SMC"). In that process, a thermosetting polyester resin filled with staple glass fiber and low profile thermoplastics, are sheeted out and thickened into a pourable paste retained between release surfaces such as polyethylene film. Chunks of the thickened paste are casually deposited around the surface of the mold by hand, and on closing the mold with heating, the paste is liquified and it, and its fiber loading, are redistributed around the mold to fill it up and form the desired molded article. In other word, the chunks of sheets of SMC represent a convenient way in which to add a liquifiable moldable material to the mold. This process is presently commercially practiced in a number of industries. Advantages of the process are the convenience of storing moldable mixture and the ease of loading a mold with the molding composition.